It is becoming even more difficult to realize high power linear amplifiers of relatively small size with an ever increasing number of channels to be amplified. Class D amplifiers are widely used because they are characterized by a relatively small power consumption and reduced size and weight. One of the drawbacks due to the use of a class D audio amplifier is the need of interposing between the power stage and the loudspeaker a filter for extracting the low-frequency spectral content (from about 20 Hz to about 20 kHz) from the output of the power stage.
The functioning principle of a class D amplifier consists in modulating a carrier of a frequency fc with a signal to be amplified of frequency fs much smaller than fc, and in demodulating the output signal generated by the power stage. A possible modulation appropriate for this objective is the PWM modulation (Pulse Width Modulation), to which reference will be made hereinafter. In this case, the modulated signal is a square wave with a fixed frequency and duty-cycle adjusted in function of the signal to be amplified.
As shown in FIG. 1, the information content relative to the amplified signal may then be extracted from the PWM modulated signal by a low-pass LC passive filter (called also demodulation filter). Preferably, a snubber network SN is connected in parallel to the load such to reduce the load voltage ripple. The core of inductors for audio filtering applications is of a material having a non-negligible hysteresis, therefore the value of the inductance L varies and depends on the current that flows through the winding. This phenomenon is even more evident when the core is relatively small (low cost).
Therefore, the filtering operation introduces a nonlinearity that directly influences the amplified signal. As a consequence, the THD (Total Harmonic Distortion) of a class D amplifier is strongly influenced by the performance of the demodulation filter, because the filter is one of the main sources of distortion in switching amplifiers.
Moreover, the LC filter is connected in series to the load and interferes with the direct control of the amplifier of the loudspeaker making the frequency response depend from the load, as shown in the Bode diagrams of FIG. 2. The frequency response becomes even less regular when the loads, such as loudspeakers, are not purely resistive.
In order to prevent a modulation of the frequency response in function of the load, it may be preferable to choose a cut-off frequency fT of the cascade low-pass filter+load larger than 20 kHz. In general the cut-off frequency fT is always chosen as a compromise between the need of dampening high frequency components of the output signals generated by the power stage and the requisite of the largest possible frequency response.
The possibility of introducing a filter inside a feedback loop may allow a reduction of the output harmonic distortion by compensating eventual nonlinearities introduced by the filter, or may allow a reduction of the costs of the reactive elements of the filter (a cost that in practice may be close to the cost of the whole amplifier), thus keeping unchanged the THD of the whole system. Moreover, by introducing a feedback of the output of the filter an enhanced control of the frequency response of the amplifier on the load may be expected, thus making it less sensible or sensitive to load variations.
However, feedbacking the system in a classic manner by using the output of the filter, as shown in FIG. 3, would be a hardly affordable way because of the strong outphasing introduced by the LC pair that imposes a relevant reduction of the loop gain of the circuit in order to stabilize the system. As a consequence, this solution may not be capable of reducing distortion nor capable of widening the frequency response.
Several feedback amplifiers are described in literature. U.S. Pat. No. 4,456,872 discloses a switching amplifier that has a voltage and a current feedback loop. The presence of a feedback current complicates the circuit structure and uses a current sensor that increases fabrication costs of the system.
The article, “A Novel Audio Power Amplifier Topology with High Efficiency and State-of-the-Art Perform”, by T. Frederiksen, H. Bengtsson and K. Nielsen, 109th AES Convention, 2000 Sep. 22-25, Los Angeles, Calif., USA, discloses a switched power audio amplifier with at least two feedback paths, one of which is connected directly to the output of the power stage according to a COM (Controlled Oscillation Modulator) technique. The presence of a feedback at the output of the power stage generates aliasing that, generally speaking, degrades the linearity performances of the system. Moreover, the functioning is based on a self-oscillating circuit (the input signal modulates the duty-cycle of the square wave at the output of the oscillator) with a variable oscillation frequency. This characteristic may degrade the system from the point of view of the EMI (ElectroMagnetic Interference) and makes difficult the synchronization of other signals with the oscillation frequency of the circuit.
The article, “High Fidelity Pulse Width Modulation Amplifiers based on Novel Double Loop Feedback Techniques”, by N. Anderskouv, K. Nielsen, M. A. E. Denmark, 100th AES Convention, 1996 May 11-14, Copenhagen, discloses a power audio amplifier with a voltage feedback loop and a current feedback loop, Besides the drawbacks due to the presence of the previously mentioned current feedback, the loop gain is unsatisfactory.
The article, “An Asynchronous switching high-end power amplifier”, by P. van der Hulst, A. Veltman e R. Groeneberg, 112th AES Convention, 2000 May 10-13, Munich, Germany, discloses a switching power amplifier with current feedback that is affected by the above mentioned drawbacks due to the current feedback.